1. Field of the Invention
This invention relates to a system for remote control of an amplifier that amplifies multi-wavelength light in a system that transmits multi-wavelength light. In particular, it relates to a remote pump system for an optical fiber amplifier.
2. Description of the Related Art
In the advanced information society that has developed in recent years, optical fiber has become widely used in transmission paths to transmit information. Optical fiber not only makes it possible to transmit larger amounts of data at higher speed, but also is superior in long-distance transmission.
However, even in optical fiber that is superior for long-distance transmission, the signal that is transmitted through the optical fiber attenuates as the transmission path becomes longer. For this reason, in, for example, a long-distance optical transmission system that connects cities or continents, normally relay nodes are established at specified intervals; at each relay node the signal is amplified and sent on to the next relay node.
A variety of types of optical amplifiers have been developed to amplify light signals. One of them is known as the optical fiber amplifier. In particular, in the 1.55 micrometer wavelength band, rare earth-doped optical fiber amplifiers into which a rare earth substance such as erbium has been injected are widely used. In a rare earth-doped optical fiber amplifier, the rare earth substance that has been injected into the optical fiber is raised to the excited state by pump light that is input separately from the signal light, and the signal light is amplified by that pump energy.
When data are transmitted between continents, particularly in high-speed communication, ocean floor cables are usually used. These ocean floor cables are normally optical fiber cables, and optical amplifiers are installed at specified intervals. That is to say, in this kind of long-distance optical transmission system, the optical amplifiers such as optical fiber amplifiers are often laid on the ocean floor.
However, if trouble occurs in an optical amplifier laid on the ocean floor or if it deteriorates, that optical amplifier must be raised to the ocean surface in order to repair or replace it, making maintenance work difficult. Meanwhile, in order to minimize the occurrence of such trouble and deterioration, such an optical amplifier is required to have far greater reliability than an ordinary optical amplifier and it is necessary to use expensive components, making the manufacturing cost very high.
Remote pump has been proposed as one means to deal with this problem. In a remote pump system, a light source that supplies pump light for the optical fiber amplifier (and the circuit that controls that light source) are installed at some distance away from the optical fiber amplifier, normally, as shown in FIG. 1A, the light source is installed at or near the transmitter or receiver. That is to say, in a remote pump system, the light source unit and the control circuit, in which it is easy for trouble to occur, are installed on the ground, and only the optical fiber components (identified in the figures as EDF=erbium-doped fibers), in which breakdown and deterioration almost never occur, are laid on the ocean bottom. For this reason, it is possible to construct a system that is easy to maintain without making an optical fiber amplifier of higher quality (reliability) than necessary, reducing the cost.
However, the amount of information transmitted through the networks has been increasing greatly. In this situation, a great deal of research and development work has been done on the technology to increase the amount of information that the transmission paths can handle. Wavelength Division Multiplex (WDM) transmission is a technology to increase the capacity of transmission paths. Multi-wavelength transmission is a method in which several light signals at different wavelengths are multiplexed for transmission through a single optical transmission path; information can be transmitted on each wavelength (channel). Recently it has been proposed to introduce such a multi-wavelength multiplex transmission system into the remote pump optical transmission system described above. One particular case in which multi-wavelength light is transmitted in an optical transmission system with a remote pump configuration is illustrated in FIG. 1B.
When multi-wavelength light is amplified using an Er-doped fiber (EDF) in a remote pump system, normally the pump light power output from a pump light source ("pump") is held at a constant value in order to measure the stability of the amplification action in the optical fiber amplifier. When pump light is supplied to the optical fiber amplifier, all of the wavelengths of the multi-wavelength light are amplified at once. That is to say, when the multi-wavelength light is being multiplexed in a plurality of channels, signals on a plurality of channels at mutually different wavelengths are amplified all at once.
However, in general the gain in an optical fiber amplifier depends on the wavelength. For this reason, if appropriate pump control is not applied to the optical fiber amplifier, the gain on the different channels on which the multi-wavelength light has been multiplexed will no longer be the same, and the light levels on the different channels will become different. In addition, in multi-wavelength transmission, the greater the number of channels that are multiplexed on one optical fiber, the greater the amount of pump energy that becomes necessary, making it desirable to control the action of the optical fiber amplifier according to this number of channels.
However, in existing remotely pumping systems, control has not been applied considering the wavelength dependence of the optical fiber amplifier gain or the number of channels that are multiplexed. For this reason, the levels on each channel deviate from one another, or the signal light level can be inappropriate causing increased noise.